This invention relates to a process for the production of an exothermically reactive composition containing one or more finely divided solid constituents, to such compositions produced by the process and to new structures of exothermically reactive compositions.
The invention is particularly useful for the production of explosive or pyrotechnic compositions in the form of microporous particles consisting of agglomerates comprising mixed particles of the constituents of the composition.
Compositions which may advantageously be produced by the process of the invention include, for example, fast-burning gas-generating charges for projectile and rocket propellants, and inflators for vehicle occupant safety restraint xe2x80x98air-bagsxe2x80x99, pyrotechnic compositions for example for pyrotechnic delay elements, flares and smoke producers; deflagrating blasting powder; and easily initiated detonating explosives. In exothermically reactive compositions the reactive constituents are preferably in ultrafine form in order to achieve faster reaction rate as a consequence of the large area of the interface between the reacting constituents.
Hitherto reactive constituents of exothermically reacting compositions, for example of explosive compositions, have been prepared in fine form by various processes and mixed together. Many of these processes have involved grinding the constituents, singly or together with other ingredients of the composition, dry or wet in a comminution device such as a ball mill. Dry grinding has been described in United Kingdom patent specification no GB 2028785 and U.S. Pat. Nos. 3,895,098, 4,243,443 and 4,376,002. Wet grinding has been described in U.S. Pat. Nos. 3,947,300, 4,999,063 and 5,223,184. A modification of the wet grinding process described in U.S. Pat. Nos. 5,143,567 and 5,223,184 involves wet grinding of ingredients in a slurry and spray drying droplets of the slurry. In these grinding processes crystal sizes less than 5 microns are difficult to produce and, if produced, the fine particles tend to re-unite and form agglomerates, thereby impairing the intimacy of mixing of the ingredients in the reactive composition, since the degree of mixing is commensurate with the largest particle present in the mixture.
An alternative process for the preparation of ammonium perchlorate in the form of small particles described in U.S. Pat. No. 3,788,095 involves spraying a solution of the material into a refrigerated apparatus where it is frozen into small droplets which are subsequently freeze-dried by vacuum sublimation to produce the desired material. The material produced by this process is in the form of agglomerates of fine primary particles. These agglomerates retain their integrity in any subsequent mechanical mixing operation and, accordingly, the intimacy of mixing of the ingredients of a reactive composition containing the material is limited by the size of the agglomerates.
It is an object of this invention to provide a method of making exothermically reacting compositions comprising at least one reactive constituent having a smaller and more uniform crystal size than that of such constituents in compositions hitherto available. A further object is to increase the ease of ignition and burning rate of gas producing compositions for air-bags, projectile and rocket propellants, pyrotechnic compositions, igniters and deflagrating blasting powders and to increase the velocity of detonation and sensitivity to initiation to detonation of blasting explosives.
In accordance with the invention a process for the production of an exothermically reacting composition containing at least one normally solid reactive constituent comprises:
forming a uniform dispersion of the ingredients of the said composition in a carrier liquid;
forming droplets of said dispersion;
feeding said droplets into a cooling medium at a temperature below the freezing point of said carrier liquid to form solidified droplets; and
freeze-drying the solidified droplets. In the freeze-drying process the frozen droplets are subjected to pressure and temperature conditions at which vapour from the carrier liquid is removed from the frozen particles by sublimation from the solid phase without any melting of the liquid. The process of the invention produces homogeneous substantially spherical, microporous particles of the exothermically reacting composition consisting of agglomerates comprising uniformly distributed primary particles of the ingredients of the composition containing pores which may be either closed or interconnected, open pores generally having a dimension of 0.2-10 microns. The porosity of the agglomerates is generally 10-80% and preferably 40-70% by volume.
Accordingly the invention also includes an exothermically reacting composition in the form of such microporous particles.
In the agglomerates the particles of solid ingredients which do not dissolve in the carrier liquid substantially retain their original size and integrity, remaining as discrete particles throughout the process, whereas crystals of ingredients which dissolve in the carrier liquid are present as ultrafine crystals. Each agglomerate contains the ingredients of the compositions uniformly distributed in the correct proportions, the degree of mixing being commensurate with the size of the primary particles. The freeze-drying step locks the primary particles in the mixed structure and prevents segregation and re-agglomeration of like particles which occurs in conventional drying processes where the liquid is evaporated from a liquid phase.
The porosity of the microporous particles will be dependent on the concentration of the dispersion and/or the rate of sublimation. Drying of the particles may be accelerated by heating the frozen droplets. Before forming the droplets, any ingredients which are insoluble in the carrier liquid are dispersed in the carrier liquid and the dispersion is maintained in a homogeneous condition by stirring if required. In some cases the dispersion will advantageously contain a thickening agent to obtain a preferred viscosity for droplet formation and to prevent segregation of the ingredients within the droplets. Ideally, however, solid constituents will be dispersed in the carrier liquid as a colloidal suspension of ultrafine particles. Further ingredients may optionally be included in the dispersion to modify the composition.
Preferably the exothermically reacting composition comprises at least one reactive ingredient which is soluble in the carrier liquid, at least a portion of this soluble ingredient being dissolved in the carrier liquid of the dispersion. After freeze-drying this soluble ingredient will be present in the microporous agglomerates in the form of ultrafine particles in uniform admixture with the remaining ingredients. When the soluble ingredient is a crystalline substance the particles will consist of ultrafine crystals.
Water soluble reactive constituents are conveniently dissolved in water but any suitable carrier liquid may be used.
Droplets of the dispersion may be conveniently formed by spraying the dispersion through one or more orifices or by projecting the dispersion from the periphery of a rotating disc or basket in known manner. Preferred droplet diameters are in the range from 50-500 microns, more preferably 75-200 microns if subsequent pressing of the freeze-drying particles is required.
The temperature of the cooling medium may conveniently be in the range xe2x88x9240 to xe2x88x92195xc2x0 C., and preferably about xe2x88x9280xc2x0 C. The cooling medium may comprise liquid, for example, liquid air or nitrogen but a gaseous medium is preferred in order to minimise distortion of the droplets and consequently optimise the free running properties of the product particles. Suitable cold gases comprise air, nitrogen, carbon dioxide, argon and helium.
The cold gas is preferably recycled through conventional refrigeration means. Alternatively the cold gas may be cooled by a recycled refrigerated fluid which may, for example, flow through a jacket surrounding a freezing chamber in which the droplets are frozen.
For freeze-drying, the solidified droplets are preferably treated in a vacuum chamber maintained at a pressure below the triple point of the solvent, which for water is 6.11 mbar. Preferably the pressure is maintained at 0.1 to 2.0 mbar. The frozen droplets in the freeze-drying chamber are preferably heated to supply the heat of sublimation of the solvent and increase the vapour pressure, without melting the particles of said reactive constituent. The solvent vapour may conveniently be condensed in contact with a cold surface, leaving the desired freeze-dried solid particles.
The microporous particles produced by this invention comprise uniform agglomerates which are much more reactive than particles of the same composition hitherto available. This has the effect of advantageously increasing the burning rate of pyrotechnic gas-producing compositions such as propellants for firing projectiles or gas-generants for inflating air-bags, increasing the velocity of detonation and ease of initiation to detonation of blasting explosive composition, and enhancing the ignitability and burning rate of deflagrating explosive blasting powders and pyrotechnic compositions. The particles are substantially free of dust, free flowing and collapse readily under pressure to form pressed grains when, for example, pressed grains are required for propellant explosives or gas-producing compositions charges in air-bags. Moreover if some of the carrier liquid, for example water, preferably about 0.5 to 5% by weight and preferably 1 to 2%, remains in the particles, high quality grains can be formed without the inclusion of a pressing agent.
This invention provides a much safer method for producing explosive and pyrotechnic compositions containing finely divided solid reactive constituents than any of the dry mixing processes used hitherto in the manufacture of such explosive compositions since only wet mixing of a dispersion of the ingredients at low temperatures are used, thereby substantially eliminating the risk of overheating by applied heat or friction which could ignite the composition.
Explosive compositions which may advantageously be prepared by the aforedescribed method include those containing alkali metal azides such as sodium azide and oxidizing salts such as nitrates, chlorates and perchlorates of ammonia and the alkali metals.
Thus a sodium azide based gas-generating composition for air-bag inflation may be prepared from a slurry comprising a solution of sodium azide in water containing oxidizing material, such as iron oxide and silicon dioxide, dispersed in the solution. The oxides, which are readily available as ultrafine particles, will remain homogeneously distributed throughout the droplets and the particles produced therefrom. The resulting sodium azide compositions are especially advantageous as they have faster burning rates and the gaseous products contain less sodium and are therefore less toxic.
An air-bag inflation composition based on ammonium nitrate may also advantageously be produced by the aforedescribed process, but in this case, a fuel, which may advantageously be a reactive polymeric binding material, and preferably a water-soluble polymeric material, for example, polyacrylamide will be mixed with a solution of ammonium nitrate and the mixture formed into droplets which are then frozen and freeze-dried. Such a composition is especially advantageous for air-bag inflation as it produces only non-toxic gas with no slag, thus enabling the usual gas filter to be omitted from the air-bag inflator assembly.
The invention also enables the manufacture of a sensitive blasting explosive or blasting agent to be prepared from non-self-explosive ingredients such as mixtures of oxidizing salt and fuel, for example, ammonium nitrate and carbon black. Hitherto such mixtures could only be initiated to detonation by very heavy priming but the product of the present invention can be initiated by a blasting cap, detonating cord or light primer.
The invention may also be advantageously used to manufacture useful deflagrating explosive powder of the black powder class. Black powder is normally made by a process which involves prolonged milling of a composition comprising potassium nitrate, carbon and sulphur but we have found that the same composition produced by the process of the present invention using carbon black instead of charcoal has useful blasting properties although it has slightly lower density than milled black powder. An alternative deflagrating explosive powder may also advantageously be prepared from potassium nitrate and a fuel/binder such as polystyrene sulphonate.